1. Field of the Invention
This invention relates to apparatus and methods for improving contrast imaging of objects which are transparent or reflective, and also vary in thickness or index of refraction, using wavefront coding.
2. Description of the Prior Art
Most imaging systems generate image contrast through variations in reflectance or absorption of the object being viewed. Objects that are transparent or reflective but have variations in thickness can be very difficult to image because the majority of the image contrast typically is derived from variations in the reflectance or absorption of the object. These types of objects can be considered “Phase Objects”. Various techniques have been developed over the years to produce high-contrast images from phase objects. These techniques allow high contrast images from essentially transparent objects that have only variations in thickness or index of refraction. These techniques generally modify both the illumination optics and the imaging optics and are different modes of what can be called “Contrast Imaging”.
There are a number of different Contrast Imaging techniques that have been developed over the years to image phase objects. These techniques can be grouped into three classes that are dependent on the type of modification made near the back focal plane of the imaging objective and the type of illumination method used. The simplest contrast imaging techniques modify the back focal plane of the imaging objective with an intensity or amplitude mask. Other techniques modify the back focal plane of the objective with phase masks. Still more techniques require the use of polarized illumination and polarization-sensitive beam splitters and shearing devices. In all of these contrast-imaging techniques modifications to the illumination system are matched to the modifications of the imaging optics.
Contrast Imaging techniques that require amplitude modification of the back focal plane of the imaging objectives we call “Amplitude Contrast” techniques. These techniques include Hoffman modulation contrast imaging (described in U.S. Pat. No. 4,062,619), edge enhancement of phase phenomena (described in U.S. Pat. No. 4,255,014), and the VAREL imaging techniques by Carl Zeiss.
FIG. 1 (Prior Art) is a block diagram 100, which shows generally how Amplitude Contrast Imaging techniques are implemented. This block diagram shows imaging an object 108 through transmission, but those skilled in the art will appreciate that the elements could just as simply have been arranged to show imaging through reflection.
Illumination source 102 and illumination optics 104 act to produce focussed light upon Phase Object 108. A Phase Object is defined here as an object that is transparent or reflective but has variations in thickness or index of refraction, and thus can be difficult to image because the majority of the image contrast typically is derived from variations in the reflectance or absorption of the object. Obviously almost any real life object is strictly speaking a Phase Object, but only objects having enough thickness or index of refraction variation to be difficult to image will require special imaging techniques.
Objective lens 110 and tube lens 114 act to produce an image 118 upon detector 120. Detector 120 can be film, a CCD detector array, a CMOS detector, etc. The various amplitude contrast techniques differ in the form of illumination mask 106 and objective mask 112. Traditional imaging, such as bright field imaging, would result if neither an illumination mask nor an objective mask were used.
FIG. 2 (Prior Art) shows a first embodiment of an illumination, mask 106a and an objective mask 112a, constructed and arranged for Hoffman modulation Contrast Imaging. Illumination mask 106a consists of two slits 202, 204 that are narrow in comparison to the diameter of the condenser aperture. Slit 204 has nearly 100% transmittance. Slit 202 contains a polarizer, and when an adjustable polarizer (not shown) is placed in the illumination path, the effect is to make this slit have a variable transmittance. When the adjustable and slit polarizers are adjusted to give extinction, the effective transmittance of this second slit is zero. In the opposite polarization configuration the effective transmittance is 100%.
Objective mask 112a is essentially the conjugate of illumination mask 106a. Objective mask 112a consists of small regions 206, 208 of absorptive power with the remainder 210 of the mask having 100% transmittance. In operation, the light that travels through illumination slits 202, 204 that is not significantly diffracted from object 108 (as for example when a phase gradient is not present) is severely attenuated by objective mask blocks 206, 208. The light that is diffracted by object 108 passes mainly through the transparent region 210 of the objective mask. In this way Hoffman modulation contrast imaging converts phase differences in the object into intensity differences in the formed images.
FIG. 3 (Prior Art) shows illumination mask 106b and objective mask 112b set for Carl Zeiss VAREL contrast imaging. Illumination mask 106b consists of an annulus 302 with 100% transmission on a fully absorptive field 304. Objective mask 112b consists of a matching partially absorptive annulus 306 on a transparent field 308. Light that passes through illumination mask 106b and is not diffracted by object 108 is severely attenuated by objective mask 112b. Light that is diffracted by object 108 passes mainly through the center of objective mask 308 unattenuated. In this way VAREL contrast imaging converts phase differences in the object into intensity differences in the formed images.
Although Amplitude Contrast Imaging techniques effectively produce high contrast images of Phase Objects, these techniques do not allow a large depth of field or control of general focus-related aberrations. A large depth of field is important when imaging objects that have a depth that is large in relation to the depth of field of the system or when making a very low cost imaging system.
There is a need to improve Contrast Imaging of Phase Objects by increasing depth of field and controlling focus-related aberrations.